This invention relates to brazing alloys and more particularly to a multi-layer composite brazing alloy having a ductile core.
Three-layer composite brazing alloy preforms consisting of outer layers of silver-base brazing alloys and a core of copper are extensively used by the mining tool industry to braze tungsten carbide tool bits to steel tool holders. In theory, when the silver-base alloys melt (in the range of 700.degree.-900.degree. C.) during the brazing process, the copper core remains unmelted. On cooling, the copper layer, being ductile, absorbs the stresses produced due to the dissimilar thermal expansion coefficients of the carbides and the steel holder, thus preventing cracking of the brazed joint. In practice, however, during the brazing operation, the operating temperature of the brazing furnace often fluctuates well beyond the preset brazing temperature. This is characteristic of induction furnaces. Such temperature overshooting can have a deleterious effect on the quality of the brazed joint if it causes any melting of the core material. An example of a multi-layer composite brazing alloy subject to this disadvantage is AMS-4764-Cu-AMS-4764, since the difference in melting temperatures of AMS-4764 and copper is approximately 150.degree. C. AMS-4764 has a composition, in percent by weight, of 9.5 Ni-52.5 Cu-38.0 Mn.
Another disadvantage of the silver-base brazing alloys is cost of the precious metal component.
U.S. Pat. No. 3,444,613 describes a preformed compact for bonding a tungsten carbide tip to a steel shank in which a mixture of Cu-Mn-Ni powder and Ni powder is sintered, cold rolled to form a strip of desired thickness and then punched to produce discs. These discs are interposed between the parts to be joined and are heated to a temperature below the melting point of Ni but above that of Cu-Mn-Ni, brazing the parts together and leaving a thin layer of Ni therebetween which absorbs stresses formed on cooling. The disadvantages of this technique are substantial shrinkage and alloying of the Ni powder with the brazing alloy.